AF at smaller apertures?

[BillDrew]

squirl033 wrote:

thanks, all... that tells me what i need to know. sounds like as long as i use a lens that opens up to at least f5.6, the AF will work fine regardless of what aperture it usesto actually takethe shot... question answered, no worries!

Not true: it really depends on how much light reachs the focusing sensor, and that depends on the amount of light on the scene as well as the f/stop. No camera, with manual or auto focus, will focus on a black cat in a coal cellar at midnight without additional lighting.

Sorry to get you back to worrying, but keep in mind that you need light to take a picture. Autofocus is likely to fail before the minimum amount of light needed to take a picture (using a tripod) with any camera/lens combination. I am starting to dosome tests, but my impression is that my KM 5d will focus in any light that I can hand-hold even with the Anti Shake and ISO3200.

[peripatetic]

Pish and tosh.

The 20D AF is head and shoulders above everything else (including the D200 if early reports are correct) until you get to the next level of Canons 1 & 5 series and Nikons 2-series.

I even have photos of black cats in coal cellars taken at midnight during a lunar eclipse (did I mention the power blackout?) with available light to prove it! :blah: The AF worked just fine.

[BillDrew]

peripatetic wrote:

Pish and tosh.

The 20D AF is head and shoulders above everything else (including the D200 if early reports are correct) until you get to the next level of Canons 1 & 5 series and Nikons 2-series.

I even have photos of black cats in coal cellars taken at midnight during a lunar eclipse (did I mention the power blackout?) with available light to prove it! :blah: The AF worked just fine.

Nice photo of the cat, but I would suggest cutting the contrast a bit.

Didn't mean to suggest that the KM 5d is the best camera in the world - that title is held by the Acme 3000. Not only can ittake a photo ofa black cat in a coal cellar, but also of a blackcat in a black hole and wirelessly transmit it via Hawkins radiation. It does work best when shooting Schrodinger's cat.

[squirl033]

Norm in Fujino wrote:

squirl033 wrote:

Well . . . maybe not entirely. As you're selecting your lenses, you might try Googling for Canon 20D + "front focus" or "rear focus"; the 20D has gotten some bad press for its tendency (with some lenses?) to focus in front of or behind the subject.

i've heard that as well, particularly if the background has a lot of large, complex shapes that distract the AF... from what i've seen of the 20D's images, though, that's not by any means a major problem.

[squirl033]

BillDrew wrote:

Not true: it really depends on how much light reachs the focusing sensor, and that depends on the amount of light on the scene as well as the f/stop. No camera, with manual or auto focus, will focus on a black cat in a coal cellar at midnight without additional lighting.

yeah... i'd kinda figured that out on my own. my main concern isn't shooting AF shots after sundown; i'm perfectly willing to focus manually at that point (though even my FZ20 will focus quite clearly with little more thanChristmas lights on a house for illumination) if need be. my main concern was for tracking and focusing on fast-moving objects... birds, aircraft, etc.

[JimC]

squirl033 wrote:

thanks, all... that tells me what i need to know. sounds like as long as i use a lens that opens up to at least f5.6, the AF will work fine regardless of what aperture it usesto actually takethe shot... question answered, no worries!

Not necessarily. You see posts from time to time from users complaining about AF hunting with cheap lenses.

Most lenses are not as sharp wide open, and some of the cheaper long zooms on the market can have problems in less than optimum lighting.

Not only do you have less light reaching the sensor through the lens with a lens that has a maximum aperture of f/5.6 if that's the max at the focal length your using, but cheaper zooms tend to be a bit soft there, too.

So, this lack of light, combined with softness wide open (and an AF camera will always focus at wide open aperture), can cause some AF hunting.

In most outdoor conditions, you'll be fine. But, AF speed can vary significantly between lenses, because of brightness, sharpness and gearing. A longer lens will also mean that you'll have more camera shake (and that can impact the AF sensors' ability to see).

All of those things combined can cause difficulties in some conditions. It's best to make sure you ask about any lenses you consider to get opinions from others on it's suitability to a given task.

There are a lot of cheap lenses on the market, and there are things other than just the maximum available aperture to consider, too.

[squirl033]

bobbyz wrote:

In good sun light according to sunny 16 rule, you will be 1/400 at f16, even overcast, you will be 1/400 at f8. FZ20 can't come anywhere near that. I know, I use FZ5 and have extensively used FZ1.

true... but the FZ20 is f2.8 throughout the zoom range, and i can get 1/500 at f8 on a sunny day, even with ISO at 100. for birds in flight, especially at any appreciable distance - say, 100 yards - the DOF is over 150 feet, even at f2.8! at a target range of 50 yards, it's still 40 feet... plenty to ensure clear focus on the subject.